This invention relates to high power devices, and in particular concerns electronic devices which a high power module contains a large capacitor storage bank. The invention more specifically concerns circuitry and techniques for avoiding severe current inrush on power up.
In high power devices such as high power RF amplifiers, a large capacitor bank is employed for supplying a level DC power to the respective load devices. When the high power device is turned off, the capacitor bank discharges. The capacitor bank is completely discharged at the time of power up. When the power switch is actuated on, the discharged capacitor bank acts as a short circuit, and a large current inrush results, measured in the hundreds of amperes. This high current draw can damage input components, and can affect other equipment.
The current inrush problems associated with equipment of this type are well known, and especially for electronic equipment which contains a rather large capacitor bank. To address this problem, circuit designers have previously attempted a number of soft-start arrangements to limit the inrush current.
One rather cumbersome approach involves complicated circuitry that gradually increases the percentage of a line voltage sinusoid that is applied to the capacitor bank. Here the percentage of the waveform is gradually increased from zero to one-hundred percent, based on the accumulated charge on the capacitor bank. This arrangement involves an additional, rather complex circuit assembly, and appreciably increases the cost of the product.
Another approach to this problem involves connecting a negative temperature coefficient (NTC) thermistor device in series in the power conductor in advance of the capacitor bank. At the moment of power up, the NTC thermistor is cold, and its resistance value is relatively high. This high resistance limits the initial inrush current. Current passing through the device heats it, and as the temperature of the NTC device rises, its resistance decreases, which prevents the NTC device from further heating. However, the NTC device itself is subjected to a high energy impulse at start up, and the presence of the device in series in the power conductor can significantly degrade the reliability of the product.
Another approach is to employ a passive component to trickle-charge the capacitor bank before full power is applied, so that the charge on the capacitor bank itself will prevent large inrush current. One possible approach to this involves a resistor, a relay, and a timer: the capacitor bank is initially powered up through a large power resistor, and after a period of time the timer actuates the relay to short out the resistor. This approach requires a large heat-dissipating resistor, and also necessitates additional safety features to deal with smoke and/or fire in the event that the power resistor remains energized when the equipment is fully operational.
An alternative approach involves using a small auxiliary transformer and a bridge rectifier to precharge the capacitor bank. This technique involves additional components, and still requires the safety features mentioned just above.